Indirect time-of-flight (ToF) three-dimensional image (3DI) image sensors are based on continuously modulated light for scene illumination, and demodulation of the received light on a pixel level during integration phases. Depth information is obtained by a calculation of pixel values from several image acquisitions with pre-defined and constant phase steps between light emission and pixel modulation. For example, four depth images may be used with discrete (congruent) phase differences of 0°/90°/180/270° to estimate the depth value for each pixel. However, because these phase differences must be congruent, the system lacks of flexibility.
Given deviations from ideal signal waveforms in illumination and pixel modulation in combination with these pre-defined phase-steps leads to systematic error components—so-called wiggling error—which needs to be calculated at least at camera assembly using an electronic calibration box which can lead to increased costs in both time and expense for manufacturers. Furthermore, the measurement accuracy is limited by the number of used phase steps and the pre-configured wiggling calibration. High speed 3D depth measurement is also not possible since at least four frame readouts are typically used, with extensive calculations for subsequent depth estimation. Also, complex phase shifters are needed to maintain highly precise, unchanging phase steps which lead to increased complexity and effort for the 3DI sensor application specific integrated circuits (ASICs).
Therefore, a depth measurement procedure that simplifies the effort, allows for high measurement accuracy or high measurement rate, and reduces costs for ToF 3DI cameras and sensor chips may be desired.